CN113281166A - Novel test method for measuring hoop elasticity modulus and Poisson's ratio of composite pipe - Google Patents

Novel test method for measuring hoop elasticity modulus and Poisson's ratio of composite pipe Download PDF

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CN113281166A
CN113281166A CN202110366444.7A CN202110366444A CN113281166A CN 113281166 A CN113281166 A CN 113281166A CN 202110366444 A CN202110366444 A CN 202110366444A CN 113281166 A CN113281166 A CN 113281166A
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strain
test piece
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CN113281166B (en
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滕锦光
谢攀
汪子豪
刘开诚
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Shenzhen Research Institute HKPU
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    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N3/00Investigating strength properties of solid materials by application of mechanical stress
    • G01N3/08Investigating strength properties of solid materials by application of mechanical stress by applying steady tensile or compressive forces
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/0001Type of application of the stress
    • G01N2203/0003Steady
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/0014Type of force applied
    • G01N2203/0016Tensile or compressive
    • G01N2203/0017Tensile
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/0058Kind of property studied
    • G01N2203/0069Fatigue, creep, strain-stress relations or elastic constants
    • G01N2203/0075Strain-stress relations or elastic constants
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/02Details not specific for a particular testing method
    • G01N2203/06Indicating or recording means; Sensing means
    • G01N2203/067Parameter measured for estimating the property
    • G01N2203/0676Force, weight, load, energy, speed or acceleration
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/02Details not specific for a particular testing method
    • G01N2203/06Indicating or recording means; Sensing means
    • G01N2203/067Parameter measured for estimating the property
    • G01N2203/0682Spatial dimension, e.g. length, area, angle

Abstract

The invention discloses a novel test method for measuring the hoop elasticity modulus and Poisson's ratio of a composite pipe, which is called as an arc test piece tensile test method. The method comprises the steps of measuring the circumferential elasticity modulus and Poisson ratio of the composite pipe by tensioning an arc test piece cut along the circumferential direction of the composite pipe, wherein two wedge-shaped self-locking clamps for fixing the end part of the arc test piece are respectively connected with a tensile testing machine through a specially designed one-way hinge and are used for absorbing out-of-plane bending deformation of the arc test piece in the initial stage of the test. In addition, the test method provides specific and clear regulations on the preparation of arc-shaped test pieces, a tensile test device, a tensile loading system, a calculation method of the hoop elastic modulus and the Poisson ratio, the application range of the test method and the like. The invention has simple operation, accurate measurement result and wide applicability, and belongs to the technical field of fiber reinforced composite material tensile test.

Description

Novel test method for measuring hoop elasticity modulus and Poisson's ratio of composite pipe
Technical Field
The invention relates to the technical field of fiber reinforced composite material testing, in particular to a novel testing method for measuring the annular elastic modulus and Poisson's ratio of a fiber reinforced composite material pipe.
Background
Fiber reinforced composite materials have been widely used in civil engineering as a concrete constraining material in recent years due to their high specific strength and excellent corrosion resistance, among which the most typical applications are: the composite pipe constrains the concrete column. Wherein, the composite material pipe can be manually manufactured by a wet paving method, and can also be directly manufactured by a composite material winding pipe. In view of production automation and good quality control of the filament winding technology, and convenience of construction, the composite winding pipe is the best choice among newly-built composite confined concrete columns.
In composite tube confined concrete columns, the fibre lay direction is typically at an angle of approximately 90 degrees to the tube axis, which means: under the action of the axial pressure, the transverse expansion of the concrete can be effectively restrained by the fibers which are arranged in the approximate annular direction, so that the strength and the ductility of the member are effectively improved.
The mode of the composite pipe for restraining the concrete column from being damaged under pressure is basically characterized in that the fibers of the composite pipe are broken along the annular direction, and the concrete is crushed. Therefore, for the composite pipe, the tensile properties in the hoop direction (including the hoop ultimate tensile strain, the hoop modulus of elasticity and the poisson ratio), especially the hoop modulus of elasticity, are decisively influenced on the stress performance of the composite pipe for restraining concrete. In the test method for measuring the elastic modulus of the composite material, besides the traditional straight strip sheet tensile test, split disc test and hydrostatic test, students in various countries around the world also put forward various test methods, but the existing methods have certain limitations in practical use, and particularly on the problem of measuring the circular elastic modulus of the composite material pipe, an accurate and convenient solution is still not available, and needs to be improved and developed. In order to solve the problem, the patent provides a simple and feasible test method aiming at measuring the annular elastic modulus and the Poisson ratio of the composite pipe, and has reliable results and wide applicability according to the existing technical conditions.
Tensile test measurement of fiber-reinforced composite Material in straight strip form tensile properties measurements are well described in many national and regional test specifications, such as Standard test method for tensile Properties of Polymer-based composites [ ASTM D3039/D3039M-14(2014) ], Standard test method for tensile Properties of fiber-reinforced composites for civil engineering Reinforcement [ ASTM D7565/D7565M-10 (2017) ], test method for tensile Properties of oriented fiber-reinforced Polymer-based composites [ GB/T3354 (2014) ], test method for elastic constants of fiber-reinforced composites [ GB/T32376 (2015) ], and so forth. However, this test method is mainly directed to continuous unidirectional fiber reinforced composite flat plates and cannot be used for composite wound tubes. For the composite pipe manually manufactured by the wet laying method, although a straight strip-shaped sheet material test piece with the same material and the same laying layer can be manufactured for a tensile test, the tensile property measured by the straight strip-shaped sheet material tensile test may overestimate the actual property expressed by the composite material in the composite material constraint concrete due to the difference of the curvatures of the two and the difference of the manufacturing methods.
In comparison, the split-disk test adopts an annular test piece, relatively completely maintains the original shape of the pipe, and is widely applied to the measurement of the hoop tensile property of the composite pipe, such as the standard test method for the apparent hoop tensile strength of plastic or reinforced plastic pipes [ ASTM D2290-16(2016) ] and the test method for the apparent initial hoop tensile strength of glass fiber reinforced thermosetting plastic pipes in plastic pipeline systems [ ISO 8521(2009) ]. In the split disk test, in order to reduce the adverse effect of the friction force between the test piece and the split disk on the test result, the test section of the annular test piece should be as close as possible to the gap between the two semicircular split disks. However, as the tensile force is applied, the two split discs are gradually separated, and an unavoidable bending phenomenon occurs on a test section of the test piece, so that the accurate hoop elastic modulus and hoop tensile limit strain of the composite pipe cannot be obtained. In order to solve this problem, many improvements have been proposed for the split-disk test method, such as moving the test section of the test piece away from the gap of the split disk to eliminate the influence of the bending of the test piece on the test result, but such a process brings another problem: the friction force between the test piece and the splitting disc enables tensile stress gradients distributed along the circumferential direction to be generated on the test piece, so that accurate tensile stress of a test section cannot be obtained, and accurate hoop tensile performance of the composite pipe cannot be measured.
Theoretically, if the influence of adverse factors such as bending and friction of the annular test piece on the determination of the hoop tensile property of the composite pipe is to be eliminated, the composite pipe internal pressure test is the best choice. The basic principle of the composite pipe internal pressure test is consistent with the ' test method for determining the fracture time under continuous internal pressure of a plastic pipeline system-glass fiber reinforced thermosetting plastic pipe ' (ISO 8521(2009) '), and the composite pipe is subjected to internal pressure application by water, oil or other liquid so as to realize annular stretching of the composite pipe. The composite pipe internal pressure test method ensures the longitudinal integrity of the test piece on one hand, and on the other hand, the composite pipe expands uniformly under the action of internal pressure, and the stress state of the composite pipe is similar to that of composite pipe confined concrete, so that the measured annular elastic modulus and Poisson ratio of the composite pipe are very reliable. However, the internal pressure test has the biggest problems that: in order to normally apply water pressure or oil pressure to the inner wall of the pipeline, it is necessary to ensure that the liquid does not leak during the pressurization process, so the sealing device at the end part is important; particularly for composite pipes with high strength and relatively small diameter-thickness ratio, high internal pressure needs to be applied to the composite pipes to obtain accurate annular material performance data. In addition, while considering the sealing effect, it is necessary to eliminate the adverse effect of the stress generated in the axial direction of the test piece due to the end seal on the test result. To solve the above two problems, the end sealing device for the internal pressure test is often designed to be complicated. In addition, the tube diameter and thickness of the composite material wound tube produced industrially or the composite material tube produced manually by a wet-laying method have certain dispersion, which often leads to the failure of the standardized sealing device in the internal pressure test.
In summary, it can be known from the analysis of the prior art that the conventional test method for measuring the circumferential material performance of the composite pipe mainly has the following three problems, so that the method cannot be applied to measuring the circumferential elastic modulus of the composite pipe:
(1) the straight strip-shaped sheet tensile test is only suitable for measuring the relevant performance of the composite flat plate and cannot solve the problem that the composite pipe has radian along the annular direction;
(2) the problem of bending or friction force which cannot be avoided in the split disc test causes a large error of the measured annular elastic modulus of the composite pipe;
(3) the test device for the internal pressure test is complicated and has poor applicability to the size deviation of the composite pipe, so that the test device is difficult to popularize and apply in practical projects.
Disclosure of Invention
The technical problem to be solved by the present invention is to provide a novel test method for measuring the circumferential elastic modulus and the poisson ratio of a composite pipe, aiming at solving the problems of inaccurate measurement, inconvenient operation and poor applicability of the circumferential elastic modulus and the poisson ratio of the composite pipe in the prior art.
The technical scheme adopted by the invention for solving the technical problem is as follows:
a novel test method for measuring the hoop elasticity modulus and Poisson's ratio of a composite pipe is used for stretching an arc-shaped test piece, and the method comprises the following steps: the tested arc-shaped test piece is directly cut along the circumferential direction of the composite pipe, and a strain gauge is pasted; two ends of the arc-shaped test piece are respectively clamped on the wedge-shaped self-locking clamp; the wedge-shaped self-locking clamp is connected with a tensile testing machine through a one-way hinge so as to realize the free rotation of the arc-shaped test piece around the one-way hinge steel shaft in the tensile process; the test comprises two stages in total; first stage of the experiment: under the stretching action, the out-of-plane bending of the arc-shaped test piece is gradually absorbed until the arc-shaped test piece is straightened; second stage of the experiment: the arc-shaped test piece is pulled until the arc-shaped test piece is stretched to the specified strain or finally damaged, and the whole test is finished; and calculating according to the tension data and the strain data measured in the linear range of the second stage of the test to obtain the hoop elastic modulus and/or Poisson ratio of the composite pipe.
Preferably, the arc-shaped test pieces are ring-shaped test pieces at least 2 heights of a composite pipe, each ring is provided with at least 2 arc-shaped test pieces, and the total number of the arc-shaped test pieces is not less than 4.
Preferably, the width of the arc test piece should not exceed 35mm and should not be less than 20mm, and the gauge length of the arc test piece should not be less than 100mm and should not be more than 200 mm. In order to ensure the transmission of the tensile force, the anchoring length of the two ends of the arc-shaped test piece is not less than 30 mm.
Preferably, the adhesive strain gauge includes: the strain gauge is accurately positioned and adhered to the inner surface and the outer surface of the arc-shaped test piece, a 1 st strain gauge and a 2 nd strain gauge are respectively adhered to two sides of the middle part of the inner surface of the arc-shaped test piece along the longitudinal direction, and a 3 rd strain gauge is adhered to the center position of the outer surface along the longitudinal direction; when the Poisson's ratio needs to be measured, besides the 1 st strain gage, the 2 nd strain gage and the 3 rd strain gage, the 5 th strain gage needs to be transversely pasted at the position, close to the 3 rd strain gage, of the outer surface of the arc-shaped test piece, the 4 th strain gage is transversely pasted at the position, corresponding to the 5 th strain gage, of the inner surface of the arc-shaped test piece, the strain gage with the gauge length ranging from 5mm to 20mm is preferably adopted, and meanwhile, the gauge length of the strain gage is not smaller than 3 times of the minimum repeated size of the fiber distribution structure of the arc-shaped test piece.
Preferably, the one-way hinge is a steel processing component, one end of the one-way hinge is a solid cylinder, and the one-way hinge is fixedly connected with the tensile testing machine; the other end of the wedge-shaped self-locking clamp is a hollow cylinder, an end rod of the wedge-shaped self-locking clamp is inserted into the hollow cylinder, and the wedge-shaped self-locking clamp is connected with the one-way hinge through an inserted steel shaft; the hollow cylinder of the one-way hinge is provided with a notch at two sides of a plane vertical to the steel shaft, and the wedge-shaped self-locking clamp can freely rotate around the steel shaft in the plane (the position of the steel shaft is a hinge fulcrum), so that the arc-shaped test piece can be smoothly straightened in the stretching process.
Preferably, the whole tensile test is carried out from the beginning of loading until the test piece is pulled to a preset strain or is damaged, the total time is not less than 30 minutes, the loading mode of the test is displacement control, and the first stage and the second stage of the test adopt different loading speeds; the first stage is a bending control stage, firstly, the distance of a loading head to be moved in the stage is determined according to the difference value between the arc length of an arc test piece between two unidirectional steel hinge shaft positions and the linear distance of the two unidirectional steel hinge shafts at the initial starting moment, and then the loading speed in the stage is calculated under the limiting condition that the loading time in the stage is not less than 5 minutes; the second stage is a stretching control stage, and the loading speed should be reduced to 0.1-0.2 mm/min.
Preferably, in the first stage and the second stage of the test, all the data such as the tension data and the strain data are continuously recorded by using the data acquisition instrument.
Preferably, the average of the data of the 1 st strain gage, the 2 nd strain gage and the 3 rd strain gage is used for eliminating the centering error of the arc-shaped test piece and simultaneously is also used for judging whether the centering error is within an acceptable range, and the centering error of the test can be calculated by the following formula:
Figure BDA0003007716670000041
Figure BDA0003007716670000042
Figure BDA0003007716670000043
in the formula:
Bythe out-of-plane bending index (%) of the arc test piece is expressed, and the increment form is adopted and is only used for error analysis in the second stage;
Bz-representing a% in-plane bending index of the arc specimen;
ε1,ε2,ε3-indicating the readings of the 1 st, 2 nd and 3 rd strain gauges, respectively;
εave-representing the average tensile strain of the arc specimen;
εc,εt-respectively representing the strain of the outer surface of the arc test piece and the strain of the inner surface of the arc test piece, which are respectively equal to epsilon3And
Figure BDA0003007716670000051
Δεave-to representAverage tensile strain increment of the second stage of the arc-shaped test piece;
Δε3-representing the mean increase in tensile strain Δ ε in the second stageaveThe corresponding 3 rd gauge reading increment;
Byand BzPreferably within + -5%.
Preferably, the hoop modulus of elasticity and the poisson's ratio of the test piece are calculated as follows:
Figure BDA0003007716670000052
in the formula:
σ — represents tensile stress (MPa);
f-represents the tensile load (N);
b-represents the arc specimen width (mm);
t-represents the arc specimen thickness (mm);
corresponding to the second stage of test loading (namely the stretching control stage), the tensile stress-tensile strain curve basically shows linearity, the slope of the tensile stress-tensile strain curve is the annular elastic modulus of the composite pipe, and in order to ensure the reliability of data, the strain increase delta epsilon in the linear range in the tensile stress-average tensile strain curve is selectedaveAt least 0.2% of the data between the two points is used to calculate the hoop modulus of elasticity of the test piece, which is the ratio of the corresponding increase in tensile stress to the increase in average tensile strain, as calculated by the following equation:
Figure BDA0003007716670000053
in the formula:
Eθ-representing the hoop modulus of elasticity (MPa) of the composite pipe;
Δεave-represents a second stage average tensile strain increase of at least 0.2%;
delta sigma-represents the mean increase in tensile strain from the second stage, Delta epsilonaveCorresponding tensile stress increment (MPa);
calculation of poisson ratio:
the poisson ratio is the ratio of the corresponding average transverse strain increase to the average longitudinal strain increase, and is calculated by the following formula:
Figure BDA0003007716670000054
Figure BDA0003007716670000055
εf=εave
vθx-representing the composite pipe hoop poisson's ratio;
εl-representing the average transverse strain of the arc specimen;
εfrepresenting the mean longitudinal strain (i.e.. epsilon.) of the curved specimenave);
ε4,ε5-indicating the readings of the 4 th and 5 th strain gauges, respectively;
Δεl-expressing and second stage mean tensile strain delta [ epsilon ]aveA corresponding average lateral strain increment;
Δεf-expressing and second stage mean tensile strain delta [ epsilon ]aveCorresponding average longitudinal strain increase (i.e., Δ ε)ave)。
As a preference, the method is applicable to a composite pipe satisfying the following conditions:
Figure BDA0003007716670000061
in the formula:
t-represents the thickness of the composite pipe;
d-represents the diameter of the composite pipe;
εu-representing a preset maximum value of the average tensile strain applied in the test or the hoop limit tensile strain of the composite pipe;
εerepresenting the second stage mean increase in tensile strain (i.e. Δ ε) used to calculate the modulus of elasticityave) Taking 0.2 percent;
in addition, in order to avoid the adverse effect of the boundary effect (the effect that the fiber in the arc test piece is cut off at the edge), the test method is suitable for the composite pipe with the fiber layering angle (the included angle between the fiber direction and the axial direction of the pipe) being more than or equal to 70 degrees; when the fiber layering angle is less than 70 degrees, the width of the test piece is preferably increased properly, and the influence of the nonlinearity of the stress-strain curve is considered.
Compared with the traditional straight strip-shaped sheet material tensile test, split disc test and internal pressure test method, the arc-shaped test piece tensile test method has the advantages that: the test device and the test method are simple to operate, accurate in measurement result and wide in applicability. The characteristics of extensive applicability mainly embody following four aspects: (1) the method is suitable for composite pipes with circular, elliptical or other curved sections; (2) the method has good adaptability to the dispersion of the geometric dimension of the measured composite material winding pipe; (3) the device can be implemented by various types of tensile testing machines or universal material testing machines; (4) in addition to typical wound composite pipes, the hoop modulus of elasticity and Poisson's ratio can be determined for composite pipes made by hand using a wet-laid process or other curved open or closed cross-section composite pipes.
Drawings
FIG. 1 is a schematic view of a curved test piece made of a composite tube according to the present invention.
FIG. 2 is a front view of an arc-shaped test piece to which a strain gauge is attached according to the present invention.
FIG. 3 is a side view of an arc test piece of the present invention with a strain gage attached.
Fig. 4 is a schematic view of a one-way hinge according to the present invention.
FIG. 5 is a schematic drawing of the tension of an arc test piece in the present invention.
FIG. 6 is a tensile stress-strain curve of the arc specimen in the tensile test of the present invention.
In the figure: 1-1 st strain gage; 2-2 nd strain gage; 3-3 rd strain gage; 4-4 th strain gage; 5-5 th strain gage; 6-arc test pieceAn inner surface; 7, the outer surface of the arc-shaped test piece; 8, unidirectional hinging; 9-wedge-shaped self-locking clamp; 10-arc test piece; 11-one-way hinge steel shaft; w is the width of the arc test piece; l0-gauge length of arc length.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention clearer and clearer, the present invention is further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
The invention provides an arc test piece tensile test method for measuring the hoop elastic modulus and Poisson ratio of a composite pipe, and the specific implementation mode and requirements of the invention are described by referring to figures 1-6 and taking some examples.
(1) Preparation of arc-shaped test piece
The tested arc test pieces 10 are directly cut along the circumferential direction of the composite pipe, specifically, the test pieces need to be taken from annular test pieces at least 2 heights of the composite pipes in the same batch of composite pipe constrained concrete members, each ring is provided with not less than 2 arc test pieces 10, and the total number of the arc test pieces is not less than 4, as shown in fig. 1. In addition, the arc-shaped test piece 10 should be cut along the circumferential direction of the composite pipe, and the edge is smooth, so that the adverse effects of the edge effect and the centering error on the test result are reduced as much as possible.
(2) Arc test piece size
In order to control the bending along the width direction (i.e. in-plane bending or in-plane bending) caused by the centering error, the width w of the arc-shaped test piece should not exceed 35mm, and meanwhile, the width of the test piece should not be smaller than 20mm in order to facilitate the pasting of the strain gauge. The scale distance l of the arc length of the arc test piece is considered in consideration of the influence of the out-of-plane bending of the arc test piece on the linear stretching of the arc test piece and the influence of the clamp pressure on the test section of the test piece0Should not be less than 100mm, and considering the large bending deformation in the initial stage of stretching and the arc length scale distance l0And should not be larger than 200mm as shown in figure 1. In order to ensure the transmission of the tensile force, the anchoring length of the two ends of the arc-shaped test piece 10 should not be less than 30 mm.
(3) Preparation of the test
Before the tensile test is started, each arc tensile test piece needs to be numbered, and the geometric data such as the thickness, the width and the like of the test piece are accurately measured and recorded according to the numbers. Then, a strain gauge is pasted on the test piece, specifically, the strain gauge is precisely positioned and pasted on the inner surface and the outer surface of the arc-shaped test piece 10, a strain gauge (i.e. the 1 st strain gauge 1 and the 2 nd strain gauge 2) is respectively pasted on the two sides of the middle part of the inner surface (the tension surface in the bending control stage) along the longitudinal direction, and a strain gauge (i.e. the 3 rd strain gauge 3) is pasted on the center position of the outer surface (the compression surface in the bending control stage) along the longitudinal direction, as shown in fig. 2 and 3. When the poisson ratio needs to be measured, besides the longitudinal strain gauge, a 5 th strain gauge 5 needs to be pasted on the outer surface of the arc-shaped test piece close to the 3 rd strain gauge 3 along the transverse direction, and a 4 th strain gauge 4 needs to be pasted on the inner surface 6 of the arc-shaped test piece along the transverse direction corresponding to the 5 th strain gauge 5, as shown in fig. 2 and 3. Preferably, a gauge length of 5mm to 20mm is used, and the gauge length of the strain gauge should be no less than 3 times the minimum repeat dimension of the fiber distribution structure of the arc test piece 10.
(4) Stretching clamp
Considering the operation simplicity of centering and fastening the test piece, the invention proposes that the wedge-shaped self-locking clamp 9 is adopted to respectively clamp two ends of the arc-shaped test piece 10, and the anchoring surface of the clamp needs to meet certain roughness so as to provide enough anchoring force. The wedge-shaped self-locking clamp 9 is connected with the tensile testing machine through the one-way hinge 8.
Because the arc-shaped test piece 10 undergoes a long section of out-of-plane bending deformation at the initial stage of tensioning until the test piece is straightened, the invention designs two unique one-way hinges 8 to respectively connect the wedge-shaped self-locking clamp 9 with the upper end and the lower end of the tensile testing machine, as shown in fig. 5. The one-way hinge 8 is a steel processing component, and one end of the one-way hinge is a solid cylinder as shown in figure 4 and is fixedly connected with the tensile testing machine; the other end is a hollow cylinder, an end rod of the wedge-shaped self-locking clamp 9 is inserted into the hollow cylinder, and the wedge-shaped self-locking clamp 9 is connected with the one-way hinge 8 through an inserted steel shaft; the hollow cylinder of the unidirectional hinge 8 is provided with a gap on each side of the plane vertical to the steel shaft, and the wedge-shaped self-locking clamp 9 can freely rotate around the steel shaft in the plane (the position of the steel shaft is a hinge fulcrum). During the stretching of the arc test piece 10, the wedge-shaped self-locking clamp 9 can rotate freely around the steel shaft, and the arc test piece 10 can be straightened smoothly.
(5) Loading system
Figure 5 illustrates a typical arc specimen tensile test procedure. The whole test process is divided into two stages, namely a bending control stage and a stretching control stage.
The first phase, the bending control phase, occurs at the beginning of the test loading, from the start of the loading until the specimen is straightened, the deformation of the specimen being dominated by bending. In the process, the tensile force of the testing machine is increased slightly, and the absolute values of the strain of the inner surface and the outer surface (namely the tension surface and the compression surface) of the test piece are correspondingly increased.
The second stage, the tension control stage, begins after the specimen is straightened until the arcuate specimen 10 is stretched to a specified strain or eventually fails, and the test ends. In this process, the increase in the film strain of the test piece gradually replaces the increase in the bending strain, and the deformation of the test piece is dominated by the stretching. At the same time, the absolute value of the longitudinal strain on the outer surface (i.e., the pressure receiving surface) of the test piece undergoes a transition from rising to falling.
The tensile test of the whole arc-shaped test piece 10 is carried out from the beginning to the moment when the test piece is pulled to a preset strain or is damaged for no less than 30 minutes, the loading mode is displacement control, and different loading speeds are adopted in the two stages. In the first stage, the distance that the loading head needs to move in the present stage is determined according to the difference between the arc length of the arc test piece 10 between two hinge points (namely, the positions of the one-way hinge steel shafts 11) and the linear distance between the two hinge points at the initial starting time, and then the loading speed in the present stage is calculated under the limiting condition that the loading time in the present stage is not less than 5 minutes. After the test enters the second stage, the loading speed should be reduced to 0.1-0.2 mm/min. In the whole process, all the tension data and the strain data are continuously recorded by using a data acquisition instrument.
(6) Data processing
Fig. 6 shows a typical tensile stress-tensile strain curve of an arc test piece in the whole process of a tensile test, wherein the specified tensile is positive, the specified pressure is negative, and the curves of the inner surface 6 (tension surface) and the outer surface 7 (compression surface) of the arc test piece are obviously divided into two stages, namely a bending control stage and a tensile control stage.
The average of the data of the three longitudinal strain gauges can eliminate the centering error of the test piece to a certain extent, and is also a basis for judging whether the centering error is within an acceptable range.
(a) Error analysis
The centering error can be calculated by:
Figure BDA0003007716670000091
Figure BDA0003007716670000092
Figure BDA0003007716670000093
in the formula:
Bythe out-of-plane bending index (%) of the arc test piece is expressed, and the method is only used for error analysis of the second stage (the stretching control stage) in an incremental mode;
Bz-representing a% in-plane bending index of the arc specimen;
ε1,ε2,ε3-readings of strain gauges 1, 2 and 3 of fig. 2 and 3, respectively;
εave-representing the average tensile strain of the arc specimen;
εc,εtrespectively representing the strain of the outer surface 7 (compression surface) and the inner surface 6 (tension surface) of the arc test piece, which are respectively equal to epsilon3And
Figure BDA0003007716670000094
Δεave-representing the average tensile strain increase of the second stage of the arc specimen;
Δε3-representing the mean increase in tensile strain Δ ε in the second stageaveThe corresponding 3 rd gauge reading increment;
Byand BzPreferably within + -5%.
(b) Calculation of the Ring modulus of elasticity
The tensile stress is calculated as follows:
Figure BDA0003007716670000101
in the formula:
σ — represents tensile stress (MPa);
f-represents the tensile load (N);
b-represents the arc specimen width (mm);
t-represents the arc specimen thickness (mm).
The second stage of the tensile stress-strain curve is basically linear, the slope of the tensile stress-strain curve is the annular elastic modulus of the composite pipe, and in order to ensure the reliability of data, the strain increase delta epsilon in the linear range in the tensile stress-average tensile strain curve is selectedaveAt least 0.2% of the data between the two points is used for calculating the hoop modulus of elasticity of the arc test piece, which is the ratio of the corresponding tensile stress increase to the average tensile strain increase, and is calculated by the following formula:
Figure BDA0003007716670000102
in the formula:
Eθ-representing the hoop modulus of elasticity (MPa) of the composite pipe;
Δεave-represents an average tensile strain increase of at least 0.2% during the stretch control phase;
delta sigma-represents the mean increase in tensile strain Delta epsilon associated with the stretch control phaseaveCorresponding tensile stress increment (MPa);
(c) calculation of Poisson's ratio
The poisson ratio is the ratio of the corresponding average transverse strain increase to the average longitudinal strain increase, and is calculated by the following formula:
Figure BDA0003007716670000103
Figure BDA0003007716670000104
εf=εave
vθx-representing the composite pipe hoop poisson's ratio;
εl-representing the average transverse strain of the arc specimen;
εfrepresenting the mean longitudinal strain (i.e.. epsilon.) of the curved specimenave)。
ε3,ε4-the readings of the 4 th and 5 th strain gauges of fig. 2 and 3, respectively;
Δεl-representing the mean tensile strain increase Δ ε over the stretch control periodaveA corresponding average lateral strain increment;
Δεf-representing the mean tensile strain increase Δ ε over the stretch control periodaveCorresponding average longitudinal strain increase (i.e., Δ ε)ave)。
(7) Application range of arc test piece tensile test
In order to ensure that the linear range of the tensile stress-strain curve is at least 0.2 percent, the test piece should meet the following conditions:
Figure BDA0003007716670000111
in the formula:
t-represents the thickness of the composite pipe;
d-represents the diameter of the composite pipe;
εu-representing a preset maximum value of the average tensile strain applied in the test or the hoop limit tensile strain of the composite pipe;
εerepresenting the incremental tensile strain in the stretch control phase (i.e. Δ ε) used to calculate the hoop modulus of elasticityave) And taking 0.2 percent.
For example, when the method is used for testing the glass fiber composite material winding pipe, the ultimate tensile strain can reach about 2%, and when the diameter of the glass fiber composite material winding pipe is 300mm, the following requirements are met:
t≤(0.02-0.002)×300=5.4mm
in addition, in order to avoid the adverse effect of the boundary effect (the effect that the fiber in the arc test piece is cut off at the edge), the test method is suitable for the composite pipe with the fiber layering angle (the included angle between the fiber direction and the axial direction of the pipe) being more than or equal to 70 degrees; when the fiber layering angle is less than 70 degrees, the width of the test piece is preferably increased properly, and the influence of the nonlinearity of the stress-strain curve is considered.
It is to be understood that the invention is not limited to the examples described above, but that modifications and variations may be effected thereto by those of ordinary skill in the art in light of the foregoing description, and that all such modifications and variations are intended to be within the scope of the invention as defined by the appended claims.

Claims (10)

1. A novel test method for measuring the hoop elasticity modulus and Poisson's ratio of a composite pipe is characterized in that an arc test piece is stretched, and the method comprises the following steps: the tested arc-shaped test piece is directly cut along the circumferential direction of the composite pipe, and a strain gauge is pasted; two ends of the arc-shaped test piece are respectively clamped on the wedge-shaped self-locking clamp; the wedge-shaped self-locking clamp is connected with a tensile testing machine through a one-way hinge so as to realize the free rotation of the arc-shaped test piece around the one-way hinge steel shaft in the tensile process; the test comprises two stages in total; first stage of the experiment: under the stretching action, the out-of-plane bending of the arc-shaped test piece is gradually absorbed until the arc-shaped test piece is straightened; second stage of the experiment: the whole test is completed after the arc test piece is pulled until the arc test piece is stretched to the specified strain or finally damaged; and calculating to obtain the hoop elastic modulus and/or Poisson ratio of the composite pipe according to the tension data and the strain data measured in the linear range in the second stage of the test.
2. The method of claim 1, wherein the arc coupons are taken from ring coupons at least 2 heights from a single composite tube, not less than 2 arc coupons are taken per ring and the total number of arc coupons is not less than 4.
3. The method of claim 1, wherein: the width of the arc test piece should not exceed 35mm and should not be less than 20mm, the gauge length of the arc test piece should not be less than 100mm and should not be more than 200mm, and the anchoring length of the two ends of the arc test piece should not be less than 30 mm.
4. The method of any of claims 1-3, wherein the attaching a strain gage comprises: the strain gauge is accurately positioned and adhered to the inner surface and the outer surface of the arc-shaped test piece, a 1 st strain gauge and a 2 nd strain gauge are respectively adhered to two sides of the middle part of the inner surface of the arc-shaped test piece along the longitudinal direction, and a 3 rd strain gauge is adhered to the center position of the outer surface along the longitudinal direction; when the Poisson's ratio needs to be measured, besides the 1 st strain gage, the 2 nd strain gage and the 3 rd strain gage, the 5 th strain gage needs to be transversely pasted at the position, close to the 3 rd strain gage, of the outer surface of the arc-shaped test piece, the 4 th strain gage is transversely pasted at the position, corresponding to the 5 th strain gage, of the inner surface of the arc-shaped test piece, the strain gage with the gauge length ranging from 5mm to 20mm is preferably adopted, and the gauge length of the strain gage is not smaller than 3 times of the minimum repeated size of the fiber distribution structure of the arc-shaped test piece.
5. The method according to any one of claims 1 to 3, wherein the one-way hinge is a steel processing member, one end of which is a solid cylinder and is fixedly connected with the tensile testing machine; the other end of the wedge-shaped self-locking clamp is a hollow cylinder, an end rod of the wedge-shaped self-locking clamp is inserted into the hollow cylinder, and the wedge-shaped self-locking clamp is connected with the one-way hinge through an inserted steel shaft; the hollow cylinder of the one-way hinge is provided with a notch at two sides of a plane vertical to the steel shaft, and the wedge-shaped self-locking clamp can freely rotate around the steel shaft in the plane, so that the arc-shaped test piece can be smoothly straightened in the stretching process.
6. The method of claim 5, wherein the total time of the first stage and the second stage is not less than 30 minutes, the loading mode of the test is displacement control, and the first stage and the second stage of the test adopt different loading speeds: firstly, determining the distance of a loading head to move in the stage according to the difference between the arc length of an arc test piece between two unidirectional steel hinge shaft positions and the linear distance of the two unidirectional steel hinge shafts at the initial starting moment, and then calculating the loading speed in the stage under the limiting condition that the loading time in the stage is not less than 5 minutes; after the test enters the second stage, the loading speed should be reduced to 0.1-0.2 mm/min.
7. The method of claim 6, wherein all of the tension and strain data is continuously recorded during the first and second stages of the test using a data acquisition instrument.
8. The method of claim 4, wherein the average of the 1 st, 2 nd and 3 rd strain gage data is used to eliminate the centering error of the arc specimen and also to determine whether the centering error is within an acceptable range, and the centering error of the test is calculated by the following formula:
Figure FDA0003007716660000021
Figure FDA0003007716660000022
Figure FDA0003007716660000023
in the formula:
Byrepresenting the out-of-plane bending index of the arc test piece in an incremental form for error separation in the second stageSeparating out;
Bz-representing an in-plane bending index of the arc test piece;
ε1,ε2,ε3-indicating the readings of the 1 st, 2 nd and 3 rd strain gauges, respectively;
εave-representing the average tensile strain of the arc specimen;
εc,εt-respectively representing the strain of the outer surface of the arc test piece and the strain of the inner surface of the arc test piece, which are respectively equal to epsilon3And
Figure FDA0003007716660000024
Δεave-representing the average tensile strain increase of the second stage of the arc specimen;
Δε3-representing the mean increase in tensile strain Δ ε in the second stageaveThe corresponding 3 rd gauge reading increment;
Byand BzPreferably within + -5%.
9. The method of claim 7, wherein the hoop modulus of elasticity and the poisson's ratio for the arcuate specimen are calculated as follows:
Figure FDA0003007716660000025
in the formula:
σ — represents tensile stress;
f-represents tensile load;
b-represents the width of the arc test piece;
t-represents the thickness of the arc test piece;
corresponding to the second stage of test loading, the tensile stress-tensile strain curve is basically linear, the slope is the hoop elastic modulus of the composite pipe, and the tensile stress-average tensile strain curve is selectedStrain increase in linear range Δ εaveAt least 0.2% of the data between the two points is used for calculating the hoop modulus of elasticity of the arc test piece, which is the ratio of the corresponding tensile stress increase to the average tensile strain increase, and is calculated by the following formula:
Figure FDA0003007716660000031
in the formula:
Eθ-representing the hoop modulus of elasticity of the composite pipe;
Δεave-represents a second stage average tensile strain increase of at least 0.2%;
delta sigma-represents the mean increase in tensile strain from the second stage, Delta epsilonaveA corresponding tensile stress increment;
calculation of poisson ratio:
the poisson ratio is the ratio of the corresponding average transverse strain increase to the average longitudinal strain increase, and is calculated by the following formula:
Figure FDA0003007716660000032
Figure FDA0003007716660000033
εf=εave
vθx-representing the composite pipe hoop poisson's ratio;
εl-representing the average transverse strain of the arc specimen;
εf-representing the average longitudinal strain of the arc specimen;
ε4,ε5-indicating the readings of the 4 th and 5 th strain gauges, respectively;
Δεl-expressing and second stage mean tensile strain delta [ epsilon ]aveA corresponding average lateral strain increment;
Δεf-expressing and second stage mean tensile strain delta [ epsilon ]aveCorresponding average longitudinal strain increments.
10. The method according to claim 1, wherein the method is applied to a clad pipe satisfying the following conditions:
Figure FDA0003007716660000034
in the formula:
t-represents the thickness of the composite pipe;
d-represents the diameter of the composite pipe;
εu-representing a preset maximum value of the average tensile strain applied in the test or the hoop limit tensile strain of the composite pipe;
εe-represents the second stage average tensile strain increase, taken as 0.2%, used to calculate the hoop modulus of elasticity;
the method is suitable for the composite pipe with the fiber layering angle more than or equal to 70 degrees.
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